Low power TDC-ADC and anger logic in radiation detection applications

ABSTRACT

A diagnostic imaging device includes a signal processing circuit ( 22 ) processes signals from a detector array ( 16 ) which detects radiation from an imaging region ( 20 ). The hit signals are indicative of a corresponding detector ( 18 ) being hit by a radiation photon. The signal processing circuit ( 22 ) includes a plurality of input channels ( 32   1   , 32   2   , 32   3   , 32   4 ), each input channel receiving hit signals from a corresponding detector element ( 18 ) such that each input channel ( 32   1   , 32   2   , 32   3   , 32   4 ) corresponds to a location at which each hit signal is received. A plurality of integrators ( 42 ) integrate signals from the input channels ( 32 ) to determine an energy value associated with each radiation hit. A plurality of analog-to-digital converters ( 44   1   , 44   2   , 44   3   , 44   4 ) convert the integrated energy value into a digital energy value. A plurality of time to digital converters ( 40 ) receive the hit signals and generate a digital time stamp. OR logic ( 36, 38 ) relays signal hits from a subset of the plurality of input channels ( 32 ) to one of the ADC ( 44 ) and one of the time to digital converters ( 40 ), the subset including more than one input channel such that more than one input channel is connected with each ADC ( 44 ) and/or each time-to-digital converter ( 40 ). A register and read out ( 25 ) reads out the locations, the digital energy values, and the digital time stamps for hit signals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/920,481 filed Sep. 1, 2010, which is a U.S. National Entryof PCT/IB2009/050755 filed Feb. 25, 2009 and claims the benefit of U.S.provisional application Ser. No. 61/036,094 filed Mar. 13, 2008, all ofwhich are incorporated herein by reference.

DESCRIPTION

The present application relates to the diagnostic imaging arts. It findsparticular application in reducing power consumption of electronicsassociated with detecting radiation, and will be described withparticular reference thereto. It is to be understood, however, that italso finds application in reducing the power consumption of arrays ofphotodetectors, and is not necessarily limited to the aforementionedapplication.

The use of silicon photo multipliers (SiPM) or multi-anodephotomultiplier tubes (PMTS) enable high performance time-of-flightpositron emission tomography (ToF-PET) detectors. These detectors havehigh temporal, spatial, and energy resolutions, and minimize pile upeffects due to the relatively small size of the detectors (a few mm).The number of read out channels to accommodate such high densitydetector arrays increases significantly over classical PMT detectors.Integrated low-power electronics conveniently accommodate the demands,especially for ToF-PET. Also, the use of neighbor logic adds complexityto the electronics. This is especially true if triggering circuitry hasto be added based on the components used. This added circuitry oftenlacks gain and delay matching, so the time stamping operationresultantly suffers.

Current time-to-digital (TDC) and analog-to-digital (ADC) applicationspecific integrated circuit (ASIC) designs perform well. While each oneconsumes relatively little power, a high density array of them consumesa relatively high amount of power, resulting in having to dissipateapproximately 1 W/cm² of heat. As the analog path from the detector tothe digitizers should be as short as possible, the detectors heat upwhen the electronics associated with the detectors are not cooledproperly. In the case of an SiPM readout, the performance of thedetectors varies significantly with temperature, so adequate cooling andthermal design is needed, adding cost and complexity to the system.

Traditional multi-channel TDC/ADCs are typically composed of amultiplicity of identical channels each having a timing branch and anenergy branch. A radiation event recordation or “hit” is typicallyproduced if the signal exceeds the trigger threshold in the timingbranch which then starts the analog integration and conversion in theenergy branch. The channel information along with the time stamp andenergy information is then sent to a sub-system for further processingand later coincidence search to form lines of response (LORs) which arethe basis of PET images. Processors that provide time stamping often usetime to amplitude conversion (TAC) which require long conversion times,reference start or stop signals, and long reset times.

Self triggered digitizers (TDC/ADC ASICs) exhibit excellent performancebut do not provide a fixed set of information for Anger Logic basedpixel identification using a single look-up table (LUT). This is due tothe fact that low energy events do not produce triggers if they do notexceed the individual triggering threshold.

The present application provides a new and improved radiation detectorarray which overcomes the above-referenced problems and others.

In accordance with one aspect, a diagnostic imaging device is provided.A signal processing circuit processes signals from a detector arraywhich detects radiation from an imaging region. The hit signals areindicative of a corresponding detector being hit by a radiation photon.The signal processing circuit includes a plurality of input channels,each input channel receiving hit signals from a corresponding detectorelement such that each input channel corresponds to a location at whicheach hit signal is received. A plurality of integrators integratesignals from the input channels to determine an energy value associatedwith each radiation hit. A plurality of analog-to-digital convertersconvert the integrated energy value into a digital energy value. Aplurality of time to digital converters receive the hit signals andgenerate a digital time stamp. OR logic relays signal hits from a subsetof the plurality of input channels to one of the ADC and one of the timeto digital converters, the subset including more than one input channelsuch that more than one input channel is connected with each ADC and/oreach time-to-digital converter. A register and read out reads out thelocations, the digital energy values, and the digital time stamps forhit signals.

In accordance with another aspect, a method of diagnostic imaging isprovided. Signals from a detector array are processed detectingradiation from an imaging region, the signals being indicative of acorresponding detector being hit by a radiation photon. Hit signals arereceived from a plurality of input channels, each input channel having acorresponding detector element such that each input channel correspondsto a location at which each hit signal is received. Signals from theinput channels are integrated to determine an energy value associatedwith each radiation hit. The integrated energy value is converted into adigital energy value. The hit signals are received and a digital timestamp is generated. Signal hits are relayed from a subset of theplurality of input channels to one of the ADC and one of the time todigital converters, the subset including more than one input channelsuch that more than one input channel is connected with each ADC and/oreach time-to-digital converter. The locations, the digital energyvalues, and the digital time stamps are read out for hit signals.

In accordance with another aspect, a method of reducing powerconsumption of a signal processing circuit which processes hit signalsfrom a detector array which detects radiation from an imaging region,the hit signals being indicative of a corresponding detector being hitby a radiation photon is provided. Hit signals are received with aplurality of input channels from a corresponding detector element suchthat each input channel corresponds to a location at which each hitsignal is received. Signals are integrated from the input channels todetermine an energy value associated with each radiation hit. Theintegrated energy value is converted into a digital energy value. Thehit signals are received and a digital time stamp is generated. Powerdraw is reduced by using OR logic to combine input channels such thatmore than one input channel is connected with each ADC and/or eachtime-to-digital converter. The locations, the digital energy values, andthe digital time stamps are read out for hit signals.

One advantage is that timing jitter is reduced.

Another advantage lies in reduced dead time of detector elements.

Another advantage lies in reduced heat generation

Another advantage is lower power consumption.

Another advantage lies in simplified circuit layout.

Another advantage lies in increased ease of cooling heat sensitivecomponents.

Another advantage lies in increased ease of supplying power to lowvoltage devices.

Still further advantages of the present invention will be appreciated tothose of ordinary skill in the art upon reading and understand thefollowing detailed description.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 is a diagrammatic illustration of a nuclear imaging device inaccordance with the present application;

FIG. 2 is a flow diagram of a triggering process where several inputchannels are encoded together for timestamping purposes;

FIG. 3 depicts another embodiment of the triggering process wheresumming, integrating, and analog-to-digital conversion are encoded forseveral input channels;

FIG. 4 depicts a possible channel encoding scheme;

FIG. 5 is a graph that depicts a probability of non-correlation ofevents as a function of distance between detector elements that areencoded together;

FIG. 6 depicts a possible circuit board wiring layout for the encodingscheme of FIG. 4;

FIG. 7 introduces the use of neighbor logic to a channel encodingscheme;

FIG. 8 depicts possible overlapping triggering zones for implementationof the neighbor logic of FIG. 7;

FIG. 9 introduces a combination of individual trigger zone time stampingas a replacement or supplement to time stamping on a summed signal;

FIG. 10 introduces discrete triggering zones to the encoding scheme ofFIG. 4;

FIG. 11 combines both ADC and TDC encoding with neighbor logic.

With reference to FIG. 1, a diagnostic imaging device 10 includes ahousing 12 and a subject support 14. Enclosed within the housing 12 is adetector array 16. The detector array 16 includes a plurality ofindividual detector elements 18. While one particular embodiment isdescribed with reference to a positron emission tomography (PET)scanner, it is to be understood that the present application is alsouseful in astrophysics, such as in gamma ray telescopes, radiography,security, industrial, and other medical applications, such as singlephoton emission computed tomography (SPECT) and x-ray. Generally, thepresent application finds use in imaging x-rays, gamma rays, or othercharged particles with high energy and spatial resolution. The array 16is arranged so that detector elements 18 are disposed adjacent animaging region 20. The detector array 16 can be a ring of detectors 18,multiple rings, one or more discrete flat or arced panels, or the like.In positron emission tomography (PET), pairs of gamma rays are producedby a positron annihilation event in the imaging region and travel inopposite directions. These gamma rays are detected as pairs, with aslight time difference (on the order of nanoseconds) between detectionsif one gamma ray travels farther to reach a detector than the other.Accordingly, in PET scanners, the detector arrays typically encircle theimaging region.

Before the PET scan commences, a subject is injected with aradiopharmaceutical. In one common exam, the radiopharmaceuticalcontains a radioactive element coupled to a tag molecule. The tagmolecule is associated with the region to be imaged, and tends to gatherthere through normal body processes. For example, rapidly multiplyingcancer cells tend to expend abnormally high amounts of energyduplicating themselves. So, the radiopharmaceutical can be linked to amolecule, such as glucose that a cell typically metabolizes to createenergy, gather in such regions and appear as “hot spots” in the image.Other techniques monitor tagged molecules flowing in the circulatorysystem.

When a gamma ray strikes the detector array 16, a time signal isgenerated. A triggering processor 22 monitors each detector 18 for anenergy spike, e.g., integrated area under the pulse, characteristic ofthe energy of the gamma rays generated by the radiopharmaceutical. Thetriggering processor 22 checks a clock 23 and stamps each detected gammaray with a time of leading edge receipt stamp. The time stamp, energyestimate and position estimation is first used by an event verificationprocessor 24 to determine if the hit data can be used for a subsequentcoincidence check. Accepted pairs define a lines of response (LORs).Because gamma rays travel at the speed of light, if detected gamma raysarrive more than several nanoseconds apart, they probably were notgenerated by the same annihilation event and are discarded. Timing isespecially important in time of flight PET (TOF-PET), as the minutedifference in substantially simultaneous events can be used to furtherlocalize the annihilation event along the LOR. As the temporalresolution of events becomes more precise, the higher the accuracy withwhich an event can be localized along its LOR. After events have beentime stamped and verified, they are passed to a register and readoutcontrol 25.

LORs are stored in an event storage buffer 26, and a reconstructionprocessor 28 reconstructs the LORs into an image representation of thesubject using filtered backprojection or other appropriatereconstruction algorithm. The reconstruction can then be displayed for auser on a display device 30, printed, saved for later use, and the like.

In one embodiment, a flash TDC is used. The TDC includes a linearfeedback shift register (LFSR) for coarse time binning, and a cascadeddelay logged loop (DLL) for fine time binning. The DLL is logged to anexternal reference frequency that controls all TDCs of the system. Thisoffers fast time to digital conversion with low dead times. Timestampvalues greater than 10⁸ counts/second can be achieved. Incoming signalsfor PET imaging normally operate at about 10³ counts/second, (e.g., forF₁₈ studies) and normally do not exceed 10⁴ counts/second for high countrate studies, using 4×4 mm² detector elements. With such a surplus oftimestamping potential available, TDCs can be multitasked to monitorseveral detector elements without sacrificing data.

By using a common digital OR signal, several detector elements that arespatially dispersed are processed by a single TDC. With reference toFIG. 2, a portion of the triggering circuitry is shown. Several inputchannels 32 ₁, 32 ₂, 32 ₃, 32 ₄ from individual detector elements feedinto respective leading edge detectors 34 ₁, 34 ₂, 34 ₃, 34 ₄. When aleading edge is detected, combined hit logic and ADC controllers 36 ₁,36 ₂, 36 ₃, 36 ₄ determine whether the detected leading edge was a validevent hit. If the leading edge is determined to be a valid hit, the hitlogic/ADC controller 36 ₁, 36 ₂, 36 ₃, 36 ₄ sends a signal to theregister and readout control 25, which identifies the channel 32 ₁, 32₂, 32 ₃, 32 ₄. The hit logic 36 ₁, 36 ₂, 36 ₃, 36 ₄ also temporarilylatches down to prevent processing of further hits until the current hitis processed and read out. At the same time it sends the signal to anencoding unit 38 where the hit signals are digitally ORed. The outputlatches a TDC 40 to timestamp the event. Still simultaneously, the hitlogic/ADC controller 36 ₁, 36 ₂, 36 ₃, 36 ₄ sends a signal to anintegrator 42 ₁, 42 ₂, 42 ₃, 42 ₄. Upon receipt of that signal, theintegrator 42 ₁, 42 ₂, 42 ₃, 42 ₄ determines an analog energy value ofthe detected event. An ADC 44 ₁, 44 ₂, 44 ₃, 44 ₄ then converts theenergy value to a digital energy value and sends it along to theregister and readout control 25. The register and readout control 25then outputs the digital amplitude, location (radiation receivingdetector,) and digital time stamp for each detected event. When the ADCcompletes the conversion and the data is read out, the hit logic 36 ₁,36 ₂, 36 ₃, 36 ₄ unlatches and opens the input channels 32 ₁, 32 ₂, 32₃, 32 ₄ to detect further events. In the above described embodiment,consumed power is reduced by almost a factor of 4.

With continuing reference to FIG. 2, four input channels 32 ₁, 32 ₂, 32₃, 32 ₄ are shown. It is to be understood that more or fewer channels 32₁, 32 ₂, 32 ₃, 32 ₄ can be tied to a single TDC 40. The depictedembodiment makes efficient use of resources, as the TDCs 40 areresponsible for approximately 90% of the power consumption. The reasonfor such marked power consumption is the demand for very small time binswith small jitter (<50 ps). By combining the time to digital conversionoperation for several detector elements with a digital OR triggerdecreases the actual number of TDCs 40 needed (by a factor of 4 in theembodiment of FIG. 2.) As the remaining TDC channels become smaller andfaster, a greater than propionate energy reduction is possible. Forexample, if the TDCs are reduced by a factor of 4-8, an energy reductionby about a factor of 5-10 is possible while improving the time stampingperformance. This allows a high end pixelated readout while consuming arelatively low amount of power (approximately 0.1 W/cm².) With thisarrangement, an entire whole body PET detector can be driven with a fewhundred Watts. The cooling system can be simplified, and temperaturesensitive parts can be regulated with greater ease. Also, the powersupply effort can be dramatically reduced for low voltage devices.

In another embodiment, as shown in FIG. 3, additional components havebeen combined to further simplify and reduce power consumption of thedetector. As in the previous embodiment, several input channels 32 ₁, 32₂, 32 ₃, 32 ₄ from individual detector elements feed into respectiveleading edge detectors 34 ₁, 34 ₂, 34 ₃, 34 ₄. The input channels 32 ₁,32 ₂, 32 ₃, 32 ₄ are summed together by a summation processor 48 andprocessed by a single integrator 42. Again, the inputs 32 ₁, 32 ₂, 32 ₃,32 ₄ are from non-contiguous detectors, because typically, only one ofthe inputs is responding to a hit and the others are substantially zero,the integrated sum represents the energy of the one detected hit. Alogic processor 36 determines whether the hit is valid, and whichchannel fired. If the hit is valid, the integrated signal is processedby an ADC 44. As in the previous embodiment, the corresponding hit logic36 ₁, 36 ₂, 36 ₃, 36 ₄ temporarily latches down when a hit is detected.This allows both the necessary chip area for the circuit layout and thepower to be reduced. The signal is time stamped by a TDC 40. Thelocation of the hit detector, the digital amplitude or energy of the hitand the time stamp for the hit pass to register and readout control 25.

For the embodiment of FIG. 3, channel dead time is determined by fourtimes the analog to digital conversion time. For a 10 bit successiveapproximation conversion, this dead time is approximately 1 μs, whichfor 10⁴ counts/second equates to about 1% dead time. This embodimentalso provides a generally acceptable signal to noise ratio despitesumming the noise of several detector channels. For both this and theprevious embodiments it is generally true that the power savings isgreater than the encoding ratio used. This is because the TDCs can beembodied on a substantially smaller geometry. This reduces thecapacitive load. Also, buffers and latches can also be designed to besmaller, faster, and can be driven with lower power.

With the two previously described embodiments, energy consumingcomponents are more effectively used. In order to multitask thesecomponents to multiple channels, the combined channels are physicallymapped in such a way to avoid cross-talk. As scintillation events causedby incoming gamma events can be scattered over several pixels, it isundesirable to map adjacent channels together into a single OR trigger.In one embodiment, the channels are physically spread apart from eachother. With reference to FIG. 4, a section of 8×8 individual detectors50 are shown. Although square detectors are shown, any shape of detectorcould also be used, such as hexagonal, round, rectangular or others. Asmentioned previously, any number can be used for the encoding, 4:1 wasillustrated in FIGS. 2 and 3, and is realized by using 2×2 sub-blocks.Another convenient ratio is 9:1 realized by using 3×3 sub-blocks.Non-square arrangements are also possible, such as 8:1, by using 2×4 subblocks. In a 3:1 encoding of hexagonal detectors, it is possible to usesub-groups of 7 or 19 detectors, arranged in a ring.

In the embodiment of FIG. 4, the array is divided into four 1:4 subblocks 52. The pixels labeled 1, 5, 33, and 37 are wired together toform the inputs 32 ₁, 32 ₂, 32 ₃, 32 ₄ of the circuits of FIG. 2 or 3.Similarly, the pixels labeled 2, 6, 34, and, 38 are connected, and soon. The grouping of the pixels is selected to minimize the possibilityof two or more detectors detecting the same event. With reference toFIG. 5, in a pixelated detector using 4×4 mm detector elements, theprobability of non-correlation of detectors is mapped for two PETscintillators 54 and 56 as a function of the distance between thedetectors. With a spread of 16 mm, approximately 99% of events for bothscintillators will be uncorrelated.

Although the distribution of connected channels could theoretically berandom, with the constraint of at least a four pixel separation, it isconvenient to arrange the connected pixels in such a way that will allowfor ease of wiring. With reference now to FIG. 6, a possible wiringscheme for an 8×8 grid of detectors with a 4:1 encoding ratio isdepicted. The circuit board is wired in two parallel planes, such as onboth sides of a typical printed circuit board. In the first plane, oneset of wirings 58 runs in one direction (from southwest to northeast inFIG. 6), while a second set 60 runs in the second plane in a seconddirection (from northwest to southeast). Each wiring contacts two pixelsand is connected to a counterpart wiring at a quad connection point,which bridges the gap between the parallel wiring planes. So in FIG. 6,pixels 5 and 33 are connected by a wire from the first set 58 and pixels1 and 37 are connected by a wire from the second set 60. The two wiresare connected at a quad connection point 62. FIG. 6 provides oneconvenient way to condense wiring for the 64 pixel array into 16 quadchannels. Other wiring schemes are certainly possible.

Neighbor logic can be used if pixel pitch and detector pitch are notidentical, and also for power savings of the above-mentionedembodiments. The neighbor logic can be overlapping or non-overlapping toneighboring detectors. With reference to FIG. 7, like reference numeralsindicate like components with FIGS. 2 and 3. A variable neighbor logicmatrix 64 selects which of channels 32 ₁, 32 ₂, 32 ₃, 32 ₄ are going tobe summed by the summer 48 for use as a timing channel. One embodimentis based on a switch matrix. If the hit logic and ADC controller 36determines that a discriminator threshold of the signal sum is exceeded,a time stamp is generated by the TDC 40 and integration and digitizationof the signals used is performed by the respective integrator 42 andADCs 44. The time stamp for the summed signal is used, so gain and delaymatching of the input signals is important to know. In one embodiment,the neighbor logic matrix 64 includes transistors, a variable gainadjustment for the sum can be implemented by local DACs 66 ₃, 66 ₂, 66₃, 66 ₄ that control the input impedance without adding a variabledelay. A delay match can be realized externally by using a layout with aconstant and almost equivalent signal path length. In an embodiment thatincludes MA-PMTs, a dynode signal can be fed into the sum, obviating aneed for further delay matching. For SiPM array readout, small timedifferences can be adjusted by the gain due to a slower impulse responseof the devices. For example, for a 30 ns rise time, a time delay shiftof 30 ps/gain % is achievable. The value of the threshold signal can beadjusted to reject more or fewer events as is desired.

FIG. 8 shows a section of a detector with overlapping trigger zones 68.the detectors marked 5, 13, 21, 30, 33, 34, 35, 36, and 37 are containedwithin multiple trigger zones. The detector setup of FIG. 8 implementedwith the neighbor logic of FIG. 7 uses four TDCs 40 and sixty-four ADCs44. As the TDC 40 consumes most of the power of the TDC/ADC, the powersavings scales with the trigger zone size. Therefore, the greater thetrigger zone size, the greater the power savings, but pile-up and deadtime effects also scale with the trigger zone size. For larger triggerzones it is possible to use ADCs 44 that allow on-chip energythresholding to reject baseline hits to reduce the amount of data to betransferred. In one embodiment, the DACs 66 could be ramped with a lowerenergy threshold as a starting value. In the above described embodiment,the power is reduced by almost a factor of 16.

In another embodiment, as shown in FIG. 9, an additional TDC value isadded for each incoming signal. In the depicted embodiment, the encodingunit 38 that uses the OR trigger passes the individual signal on to anindividual signal TDC 70. This embodiment provides individual timestamps for each signal coming from a zone exceeding the lower triggerthreshold. No exact gain and delay matching is needed, as the TDC valueon the summed signal (coming from the TDC) is optional and time stampingcan be realized by weighting the individual timestamps of the zone. Thetrigger threshold on the summed signal can be relatively high to rejectscattered events. The encoding zones are defined as discussedpreviously, with sufficiently disparate physical correlation. FIG. 10depicts an encoding scheme similar to that of FIG. 4, and adds smallertriggering zones 68 than in FIG. 8. Such an embodiment of 64 detectorswould use 16 TDCs 70 for 16 triggering zones 68 (with a TDC 40 on thesum being optional) and 64 ADCs 44.

Referring again to FIGS. 9 and 10, if a summed trigger of a zone 68exceeds the threshold (e.g. detectors marked 1, 2, 9, and 10) thecorresponding integrators 42 ₁, 42 ₂, 42 ₃, 42 ₄ and ADCs 44 ₁, 44 ₂, 44₃, 44 ₄ are started and the corresponding TDC values are latched. Here,the pixels 1, 5, 33, and 37 feed one ORed TDC 40, the pixels 2, 6, 34,and 38 feed another TDC 40, and so on. The corresponding TDC values aretransferred to the readout controller 25 together with the channel ID ofwhich channel fires. Alternatively, the timestamp value of the summedsignal can also be stored. In the above described embodiment, the poweris reduced by almost a factor of 8. This embodiment uses slightly morepower, but no exact gain and delay matching is required. Individual TDCand ADC values are provided within the trigger zone for ToF timestampingand allowing Anger logic based pixel identification by providing atleast four neighboring detector values.

With reference now to FIG. 11, another embodiment is shown. If the ADCencoding is chosen to be similar to the TDC encoding, further chip areaand power can be saved while maintaining the trigger zone readout. Basedon the pixel mapping, the number of TDCs 40 and ADCs 44 required toperform the task scales with the encoding ratio. In this embodiment, a4:1 encoding is shown, which uses 16 TDCs 40 and 16 ADCs 44 for 64 inputchannels 32 (detector pixels) while guaranteeing energy and timestampvalues for each trigger zone (68 in FIG. 10). This embodiment uses theanalog sum of several detector inputs for the energy channel. The ADCencoding leads to a lower number of necessary ADCs.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

Having thus described the preferred embodiments, the invention is nowclaimed to be:
 1. A diagnostic imaging device comprising: a signalprocessing circuit which processes hit signals from a detector arraywhich detects radiation from an imaging region, the hit signals beingindicative of a corresponding detector being hit by a radiation photon,the signal processing circuit including: a plurality of input channels,each input channel receiving hit signals from a corresponding detectorelement such that each input channel corresponds to a location at whicheach hit signal is received; a plurality of integrators for integratingsignals from the input channels to determine an energy value associatedwith each radiation hit; a plurality of analog-to-digital converters(ADCs) for converting the integrated energy value into a digital energyvalue; a plurality of time-to-digital converters (TDCs) that receive thehit signals and generate digital time stamps; OR logic that relayssignal hits from a subset of the plurality of input channels to one ofthe ADCs and one of the TDCs, and the subset includes more than oneinput channel, the more than one input channels of the subset arenon-adjacent input channels and the more than one input channels areconnected with one of the ADCs and/or one of the TDCs; and a registerand read out which reads out the location, the digital energy value, andthe digital time stamp for each hit signal.
 2. The diagnostic imagingdevice as set forth in claim 1, wherein the TDC includes a linearfeedback shift register (LFSR) for coarse time binning, and a cascadeddelay logged loop (DLL) for fine time binning.
 3. The diagnostic imagingdevice as set forth in claim 2, wherein the DLL is logged to an externalreference frequency.
 4. The diagnostic imaging device as set forth inclaim 1, wherein the subset of the plurality of input channels includesindividual input channels that are connected to detectors which arephysically separated from each other.
 5. The diagnostic imaging deviceas set forth in claim 4, wherein the individual input channels arespaced at least two pixels apart from each other.
 6. The diagnosticimaging device as set forth in claim 1, wherein the signal processingcircuit further includes a circuit board that includes wiring in atleast a first plane and a second plane parallel to the first plane. 7.The diagnostic imaging device as set forth in claim 1, furtherincluding: a plurality of leading edge detectors which determine leadingedges of the received hit signals; and a plurality of hit logiccontrollers which receives input from the detectors and determines avalid hit, and for a valid hit simultaneously sends a signal whichidentifies the channel of the valid hit, and a signal to the integratorto determine the digital energy value.
 8. The diagnostic imaging deviceas set forth in claim 1, wherein the detector array further includes: aplurality of detector elements each connected to one of the inputchannels which include a spread of at least 16 mm between detectorelements corresponding to input channels in the same OR logic subset. 9.The diagnostic imaging device as set forth in claim 1, wherein thedetector array further includes: a plurality of detector elementsarranged spatially into sub-blocks of adjacent detectors, each detectorelement connected with a corresponding input channel and the subset ofmore than one input channel includes channels each selected fromseparate sub-blocks.
 10. A diagnostic imaging device comprising: anarray of detectors, each detector generating a hit signal in response tobeing hit by a radiation photon from an imaging region; a plurality ofsignal processing circuits which processes the hit signals from thedetector array, each of the signal processing circuits including: aplurality of input channels, each input channel being connected to acorresponding detector element, the corresponding detector elementsbeing non-adjacent and separated sufficiently far that none of thedetector elements connected to a common signal processing circuitgenerate a hit signal in response to a common radiation photon; an eventverification processor that applies verification criteria to detectorchannel hits to determine valid hits; a plurality of integrators forintegrating signals from the input channels to determine an energy valueassociated with each radiation hit; a plurality of analog-to-digitalconverters (ADCs) for converting the integrated energy value of thevalid hits into a digital energy value; at least one time-to-digitalconverter (TDC) that receives the hit signals and generates digital timestamps for the valid hits, such that digital energy values and digitaltime stamps are generated for the valid hits, such that the ADC and theTDC are shared by detectors that are non-adjacent detectors; a registerand read out which reads out a location, the digital energy value, andthe digital time stamp for each hit signal; and an event storage bufferfor storing valid time stamped events; and a reconstruction processorfor reconstructing valid events into an image representation of asubject in the imaging region.
 11. The diagnostic imaging device as setforth in claim 10, wherein each input channel is connected to one of theADCs and a plurality of input channels are connected to each TDC.
 12. Adiagnostic imaging device, comprising: a signal processing circuit whichprocesses hit signals from a detector array which detects radiation froman imaging region, the hit signals being indicative of a correspondingdetector being hit by a radiation photon, the signal processing circuitincluding: a plurality of input channels, each input channel receivinghit signals from a corresponding detector element such that each inputchannel corresponds to a location at which each hit signal is received;an event verification processor that applies verification criteria todetector channel hits to determine valid hits; a plurality ofintegrators for integrating signals from the input channels to determinean energy value associated with each radiation hit; a plurality ofanalog-to-digital converters (ADCs) for converting the integrated energyvalue of the valid hits into a digital energy value; a plurality oftime-to-digital converters (TDCs) that receive the hit signals andgenerate digital time stamps for the valid hits, such that digitalenergy values and digital time stamps are generated for the valid hits;a register and read out which reads out the locations, the digitalenergy values, and the digital time stamps for hit signals; and an eventstorage buffer for storing valid time stamped events; a reconstructionprocessor for reconstructing valid events into an image representationof a subject in the imaging region; and a neighbor logic matrix thatreceives hit signals from the input channels and selects channels to becombined to create a timing signal to be conveyed to one of thetime-to-digital converters to generate the time stamp.
 13. Thediagnostic imaging device as set forth in claim 12, wherein the detectorarray further includes: a plurality of variable gain and time matchingcircuits to adjust the hit signals on the plurality of inputs before thehit signals are analyzed by the neighbor logic matrix.
 14. Thediagnostic imaging device as set forth in claim 12, wherein there is oneADC for each input channel.
 15. The diagnostic imaging device as setforth in claim 12, wherein the signal processing circuit furtherincludes a summer that sums a plurality of input channels.
 16. A methodof diagnostic imaging comprising: processing signals from a detectorarray, each detector of the array including a plurality of detectorelements responding to a radiation photon from an imaging region bygenerating a corresponding signal, the detector elements being separatedsuch that the detector elements are not responding to a common radiationphoton; receiving signals on a plurality of input channels, each inputchannel connected to a corresponding detector element; integrating eachsignal received on the input channels to determine an energy value;converting the integrated energy values into a digital energy value withanalog-to-digital converters (ADCs); generating a digital time stampwith a time-to-digital converter (TDC) for the signals received on aplurality of the input channels such that more than one input channel isconnected with each TDC; and reading out the location, the digitalenergy value, and the digital time stamp for each hit signal.
 17. Themethod as set forth in claim 16, wherein there is one ADC for each inputchannel and a single TDC is shared by a plurality of input channels. 18.The method as set forth in claim 16, further including: comparingsignals from the input channels with a neighbor logic matrix anddetermining which of the signals will be passed on for a summationoperation.
 19. A method of reducing power consumption of a signalprocessing circuit which processes hit signals from a detector arraywhich detects radiation from an imaging region, the hit signals beingindicative of a corresponding detector being hit by a radiation photon,the method comprising: receiving hit signals with a plurality of inputchannels from a corresponding detector element such that each inputchannel corresponds to a location of a detector at which each hit signalis received; integrating signals from the input channels to determine anenergy value associated with each radiation hit; converting theintegrated energy value into a digital energy value; receiving the hitsignals and generating a digital time stamp for each hit signal;reducing power draw by using OR logic to combine non adjacent inputchannels such that more than one input channel is connected with eachADC and/or each time-to-digital converter and the combined inputchannels are non-adjacent; and reading out the location, the digitalenergy value, and the digital time stamp for each hit signal.
 20. Themethod as set forth in claim 19, wherein generating the digital timestamp includes generating the time stamp for the hit signals received ona plurality of the inputs with a common time-to-digital converter.